4-Probe micropatterning and electrical measurements across individual grain boundaries in electroceramics

A novel approach combining conventional contact- and projection-lithography techniques has been devised to implement microelectrodes, down to submicron size, across isolated site-specific features as small as 2-3 micron. For both ex-situ and in-situ electrical characterization, such features of interest are isolated interfaces and grain boundaries in electroceramics and multilayer devices, including those in as-prepared TEM specimen. The procedure has been discussed for implementing 4-probe microelectrodes across several individual isolated grain boundaries of a commercial ZnO varistor containing 2-10 micron size grains. In addition, we discuss the results from dc 4-probe I-V and ac 2-probe impedance measurements across individual grain boundaries, and dc 2-probe I-V and ac 2-probe impedance measurements from grain interiors isolating these grain boundaries. Over and above the generally observed properties of isolated grain boundaries, the measurements reveal (1) inhomogeneity and applied-bias-polarity dependent asymmetry in the nonlinear I-V characteristics of grain boundaries, (2) possible presence of non-ohmic electrode-ceramic contact resistance in 2-probe measurements, and (3) a gradual process of irreversible degradation of the nonlinear I-V behavior with respect to thermal runaways upon application of a dc bias across isolated grain boundaries.